This invention relates to a signal noise/defect compensator applicable for use in a system for reproducing recorded or broadcast television image information.
Defects in television image information signals produce objectionable picture characteristics in the reproduced image in the form of bright or dark lines or spots. In broadcast TV signals these defects arise from RF interference which impose noise impulses on the transmitted signals. A familiar manifestation of such noise are the bright flashes which occur on the TV display when an electric motor such as a hair dryer is operated in the vicinity of a TV receiver. With respect to video signal recovered from a recorded medium, e.g., playback of a VTR or a video disc, defects arise from loss of portions of the recorded signal due to defects in the record. These defects are referred to in the recording arts as dropouts and produce streaks in the displayed picture. Whether the defects arise from noise or dropouts they are susceptible of detection prior to display of the image so that corrective action may be taken to reduce objectionable manifestations.
Television image information is generally redundant line to line. It is therefore possible to substitute segments of signal from adjacent image lines in place of defective signal segments. Known defect compensators of this type, as shown in U.S. Pat. No. 2,996,576, store image information from a previous line and insert the stored previous line of information, or a portion thereof, into the video signal when a signal defect occurs. This process is relatively simple for black and white TV signal but becomes more complicated with respect to composite color signals. Color TV signals include a wide band luminance signal and a phase modulated color subcarrier. The color subcarrier, however, occurs with a 180 degree phase difference from line to line. This phase difference precludes direct signal substitution between adjacent lines for defect compensation purposes because the substituted color information would be incorrectly reproduced. Fuhrer in U.S. Pat. No. 4,272,785 filed June 1, 1979 and assigned to the present assignee, solved the color phase reversal problem by substituting an average signal from a previously stored line. This is accomplished by averaging signal from two picture points displaced from the defective point by approximately .+-. one-half of the color subcarrier period. This results in a signal having a color subcarrier in phase with an adjacent line and a luminance signal which approximates the average of the luminance signal over a color subcarrier period. Bolger et al., U.S. Pat. No. 4,122,489 (incorporated herein by reference) produced a similar result by generating a substitution signal which is the average of four picture points, two taken from a succeeding image line and two from a preceding image line to the image line currently being displayed.
The Bolger et al. apparatus comprises two serially connected 1H delay lines. The signal normally displayed is acquired from the output of the first 1H delay line, and then delayed by a one-half color subcarrier period in a third delay element. The substitution signal is generated by first summing the input and output video signal of the serially connected two 1H delay lines, the resultant signal being the sum of two vertically aligned picture points from a first and third horizontal line. The summed signal is delayed one color subcarrier in a fourth delay circuit. The summed signal and the summed signal delayed by one color subcarrier period are then summed to produce the substitution signal. The fourth delay element temporally aligns two picture points along the horizontal lines separated by one color subcarrier period and thereby effects the summing of a total of four picture points. The total summed signal is then divided by four to produce a signal equal to the average signal of the four picture points. Note the third delay element creates a 180 degree phase delay in the color signal normally displayed rendering it in phase with the color signal of the substitution signal.